1. Field of the Invention
The present invention relates to conversion of a waste product into a useful building product, specifically, the conversion of bottom ash from a coal burning power plant into a high quality aggregate.
2. Description of the Prior Art
Steam generating power plants utilizing coal as fuel must dispose of waste cinders produced by burning of the coal. In a typical power plant, a substantial portion of the ash produced falls through a grate in the steam boiler, and this ash or cinder is known as "bottom ash". Normally, power plants use water as a vehicle to move the cinders or bottom ash from the plant to settling ponds for subsequent rehandling. A common method of rehandling of the settled-out cinders involves the use of excavation equipment to load the material into trucks and haul it to a disposal area which may be located a considerable distance away. Due to the lightness of the cinder and the fineness of a portion of the particles making up the ash, special handling is required to prevent dust problems. Often, the ash must be disposed of by using it in a land-fill operation whereby the cinders and fine particles are incorporated into other materials such as clay, shale, etc. to prevent dusting and related problems. Such ash disposal is very expensive, amounting to millions of dollars per year in this country.
Cinders, such as found in bottom ash from coal burning power plants, have been used for many years in the production of masonry units and lightweight concrete, and various methods for providing optimal sized cinders, such as crushing and screening, have been used. Normally, the screening takes place in the dry state. Dry screening ordinarily requires a discontinuous process, i.e., dry bottom ash usually cannot be conveniently conveyed to a processing plant on a continuous basis, such as through a pipeline. Also, dry screening has been found to be disadvantageous in grading the smaller particles found in bottom ash. In particular, some of the particles comprising bottom ash are very fine, i.e., particles which will pass 100 or 200 mesh. It has been found that a certain proportion of these fine particles are desirable as an aggregate component, as the fine particles help fill interstices between the coarser particles in the building material matrix. Ordinary bottom ash as produced in a power plant, however, includes a much greater proportion of fine particles than is desirable. Such proportion of fine particles is generally on the order of 20%. When such proportion of fine particles is used in a building material, a large amount of cement is needed to coat and bond the fine particles. The fine particle component also contributes to problems in forming of building blocks, contributes to problems in achieving uniformity in color and texture in finished building blocks, results in a long curing time, and results in a relatively heavy block. Another disadvantage of excessive fines is a problem known as "flash set" in which the fines draw water from cement causing overly rapid setting of the cement and resulting in cracking and weakening of the concrete. Although this can be overcome by adding more water and cement and by increasing the mixing time, this results in increased costs and is disadvantageous.
Screening is a generally unsatisfactory method of efficiently grading large quantities of these very fine particles from the bottom ash. Thus, no effort is normally made to remove the fine particles from the bottom ash before using it as an aggregate. The aforementioned dry screening process is ordinarily used only for separating oversized particles from the bottom ash.
It is known to dispose of the waste products of coal burning power plants by forming an aqueous slurry of the waste products, particularly fly ash, and then treating the slurry in such a way that a useful end product will be formed such as mineral wool, or aggregate material. Such processes are disclosed in U.S. Pat. No. 3,962,080 to Dulin et al and U.S. Pat. No. 3,642,445 to Muter et al. Such processes, however, also directly or indirectly involve the treatment of stack gases and, as such, either involve specialized equipment in the power plant or are applicable to only certain types of power plants. Also, these processes involve treatment of the waste product by subjecting it to very high temperatures, again involving special equipment and considerable expense.
3. Objects of the Invention
It is therefore an object of the present invention to provide a process and apparatus for producing a cinder aggregate from waste products of power plants which process and apparatus are readily compatible with virtually all types of coal burning power plants which produce bottom ash without substantial modification to the power plant.
It is a related object of the invention to provide a process and apparatus for disposing of bottom ash from a power plant and at the same time continuously producing cinder aggregate from the bottom ash at a processing plant without requiring special processing of the bottom ash at the power plant.
It is a related object of the invention to provide a process and apparatus for producing a cinder aggregate from bottom ash in which the bottom ash is continually removed from the power plant in an aqueous slurry and in which the operation of the power plant is not otherwise affected by the ash removal and aggregate production.
It is yet another object of the invention to provide a wet classifying system for treating bottom ash.
It is a further object of the invention to provide a process and apparatus for producing a uniform and lightweight aggregate from cinder containing ash.
It is a further object of the invention to provide a process and apparatus for producing an aggregate material for use in building materials, such as masonry units or pavement, wherein the aggregate material is of very high quality.
It is another object of the invention to provide a building block having an improved material composition.
It is a more specific object of the invention to provide a process and apparatus for producing an aggregate material from cinder containing ash, the use of which aggregate in a masonry unit: (a) permits a lower than usual cement content per masonry unit produced without loss of strength; (b) effects reduction of waste or cull units; (c) results in uniformity of color and texture in the finished units; (d) effects lower unit weight of the finished masonry unit thereby resulting in transportation and labor savings; and (e) results in less curing time for the "green" masonry units.
It is another object of the invention to provide a process and apparatus for disposing of power plant bottom ash and converting it into a high quality aggregate in which waste water from the conversion process is recycled back to the power plant for reuse in conveying bottom ash to the processing plant, in which a bypass arrangement is provided at the processing plant to accommodate shut downs of the processing plant without affecting continuous disposal of bottom ash from the power plant, and in which screened-out oversized cinders in the processing plant are crushed and reprocessed.
Still other objects and advantages of the present invention will be apparent from the following description and from the drawing.